This invention concerns an electric arc furnace to melt steel with alternative sources of energy and concerns also the relative operating method.
This invention is applied to the field of electric arc furnaces employed to melt steel and alloys thereof and has been conceived and designed to optimise the efficiency and output of the plant by using alternative sources of energy in a suitable manner.
The invention is applied to furnaces working with direct current and to furnaces working with alternating current.
The furnaces to which the invention is applied may have a tapping channel and a tapping hole without the positioning of the tapping hole being in any way restricted.
This is a typical preferred application for furnaces which work with the method of the "pond", that is to say, with a liquid heel always present.
The invention has the purpose of achieving significant savings of the energy consumed in feeding the furnace and of the melting times.
The invention is also particularly suitable for the melting of completely cold charges.
The invention is employed for charging the furnace with scrap by means of skips, or for continuous charging of scrap, or for continuous charging with pre-reduced material, or for mixed charges.
The methods of the state of the art for the melting of metals include the process of injecting gaseous elements into the furnace so as to accomplish a reaction of oxidation with the carbon and with the other oxidizable chemical elements in the molten metallic mass.
The oxidation reaction enables the intrinsic potential of energy of the carbon and of the other chemical elements to be recovered by means of an exothermic oxidation reaction.
The gaseous elements injected may consist of air enriched with oxygen or even of pure oxygen. These gaseous elements may be blown onto the molten metal by means of lances positioned above the surface of the bath and/or by nozzles or tuyeres placed on the bottom of the furnace.
U.S. Pat. No. 3,459,867 teaches the use of burners delivering oxygen in carefully calculated quantities and therefore with an excess of oxygen. These burners act along a circumference positioned between the electrodes and the sidewall of the furnace and work by following the movement of the metal.
U.S. Pat. No. 2,909,422 teaches the delivery of oxygen and other substances through a hollow electrode. The state of the art therefore includes the injection of additives and powdered coal through hollow electrodes placed in the roof of the furnaces. These additives and powdered coal supplement and replace the carbon used in the oxidation reaction and enable the contribution of energy provided by that reaction to be kept high.
GB-A-2,115,011 teaches the blowing of a stirring and processing gas from below the bath of molten metal, the introduction of solid carbonaceous material from above and the use of tuyeres positioned on the sidewall and delivering mixtures of gases towards the bath.
GB-A-1,421,203 teaches the delivery of oxygen and other gases from the bottom in a zone between the electrodes and the sidewall of the furnace.
FR-B-2,208,988 teaches the delivery of gases from above downwards into the furnace and also the delivery of gases from below the scrap and/or bath of molten metal into the zone of the electrodes and also between the electrodes and the sidewall of the furnace. This document discloses also in the sidewall one or more lances which deliver oxygen and/or other gases, in a configuration following movement of the metal, between the electrodes and the sidewall of the furnace.
EP-BI-0257450 discloses a method which includes a plurality of lances for oxygen, or mixtures of oxygen, working between the electrodes and the inner walls of the furnace and also a plurality of nozzles arranged below the molten bath in the zones where the lances work.
These nozzles positioned on the bottom are also employed to stir the bath and to make uniform the blowing action of the lances since, even when supersonic-type lances are used, the jet of the combustion gases cannot reach a depth greater then 20-30 cms. in the molten bath unless non-reusable throwaway lances are employed.
The nozzles and tuyeres of the state of the art work typically at pressures up to 60 bar and normally between 5 and 20 bar.
As can be seen in U.S. Pat. No. 3,902,889 and EP-BI-0257450, these nozzles and tuyeres positioned on the bottom have an oxygen emission diameter between 3 and 6 mm. and are advantageously of a two-pipe type with a central pipe emitting O.sub.2 and an annulus emitting hydrocarbons and/or inert gases for cooling purposes.
The employment of tuyeres to blow from the bottom oxygen combined, for instance, with different fluids such as argon, nitrogen and methane enables the oxidation reaction to be improved and made uniform.
The technology of the tuyeres enables a better homogenization of the liquid steel to be achieved, thus making possible intensive working in a "long arc" condition without generating disuniformity of temperature and also making possible a shortening of the melting time and still other advantages.
In the state of the art the blowing of gases from the bottom is carried out at a relatively high pressure with relatively low rates of flow of the gases through a plurality of tuyeres, generally of a small diameter, distributed on the bottom.
The regulation of the blowing during the various working cycles is also carried out on the rate of flow of the gases; the pressure can be varied, but only as a result of the variation of the rate of flow.
In the state of the art the tuyeres are positioned substantially symmetrically on the hearth of the furnace and often correspond to the oxygen emission lances.
Such an arrangement of the tuyeres and lances delivering oxygen and other gases does not lead to any improvement in the efficiency and output of the melting plant.